When you can't keep track of your sock yarn.
I was looking online today at Little Knits. I was starting to add some sock yarn to the cart. Then, something started to look familiar. I came pretty close to buying another skein of the pink/blue/brown. (Fly Designs' Monarch) It looked eerily familiar. You know why? I had some at home already. Glad I caught myself. I'll add this to my stash on Ravelry.